[Feature] Allow injecting custom environ variables in the pre-commit flake check

[sunziping2016]

Background

I'm using the flakeModule in my own homelab repo. I turned on pre-commit.check.enable.

https://github.com/tillycode/homelab-v2/blob/5562ca44dbb97e295d361728b93f7c709608ea9b/flake/pre-commit.nix#L46-L59

In the example above, I wrote a custom commit hook flat-flake in Python. Its source code is placed within the repo. To prevent devshell from rebuilding whenever there's a change in the source code, I use editable mode, which relies on an environment variable pointing to the root of the source code. You can see I use PRJ_ROOT, which follows the convention of numtide/devshell.

However, when running nix flake check, the PRJ_ROOT environment variable is unset, causing error. I set the environment in the pre-commit.settings.package to circumvent the issue, which is quite ugly.

Feature Request

I hope there is an option, such as pre-commit.check.preRunScript, for me to prepare some environment variables in the buildPhase of run derivation

git-hooks.nix/modules/pre-commit.nix

Lines 86 to 118 in f799ae9

	run =
	mkDerivation {
	name = "pre-commit-run";
	src = cfg.rootSrc;
	buildInputs = [ cfg.gitPackage cfg.package ];
	nativeBuildInputs = enabledExtraPackages
	++ lib.optional (config.settings.rust.check.cargoDeps != null) cargoSetupHook;
	cargoDeps = config.settings.rust.check.cargoDeps;
	buildPhase = ''
	set +e
	# Set HOME to a temporary directory for pre-commit to create its cache files in.
	HOME=$(mktemp -d)
	ln -fs ${cfg.configFile} ${cfg.configPath}
	git init -q
	git add .
	git config --global user.email "you@example.com"
	git config --global user.name "Your Name"
	git commit -m "init" -q
	if [[ ${toString (compare cfg.installStages [ "manual" ])} -eq 0 ]]
	then
	echo "Running: $ pre-commit run --hook-stage manual --all-files"
	${lib.getExe cfg.package} run -c ${cfg.configPath} --hook-stage manual --all-files
	else
	echo "Running: $ pre-commit run --all-files"
	${lib.getExe cfg.package} run -c ${cfg.configPath} --all-files
	fi
	exitcode=$?
	git --no-pager diff --color
	# Derivations must produce an output
	mkdir $out
	[ $? -eq 0 ] && exit $exitcode
	'';
	};

If it is available, I can just set

{
  pre-commit.check.preRunScript = ''
    export PRJ_ROOT="$PWD"
  '';
}